Fluororesin films have excellent weatherability, transparency, mechanical properties, moisture-proof property and, therefore, are used as a covering material for agricultural houses, roof film material, window material, solar battery surface protection material, light collection panel surfacing, display board surfacing, exterior building material and the like.
The sunlight can pass through them to illuminate rooms in the daytime without lamps. For environmental consciousness, the natural light illuminates interior showpieces with natural colors and the sky viewed from the interior provides a sense of openness so that they are greatly demanded particularly in the field of building material such as roof film and window materials.
On the other hand, the sunlight is sometimes harmful because of ultraviolet rays and infrared rays (heat rays). For example, strong irradiation of ultraviolet rays may cause dermatitis on a person and deteriorate synthetic substances. The infrared rays greatly irradiating into rooms may damage plants grown in the greenhouse and make a person uncomfortable with high temperatures.
To solve these problems, JP H09-205898 A discloses an infrared-ray-blocking agricultural sheet made of synthetic resin containing dispersed inorganic particles of tin oxide or the like. JP 2002-69258 A discloses a fluororesin film containing metal oxide particles coated with amorphous silica. JP 2004-25586 A discloses a heat-ray-blocking fluororesin composite sheet for roof materials containing infrared-ray-blocking inorganic particles on at least one surface.
JP 2001-171060 A discloses a colored or colorless transparent biaxially-oriented polyester film to be pasted on windows, containing near-infrared absorbent in the interlayer. JP 2008-181838 A discloses a transparent conductive laminate made by modifying the transparent color of transparent conductive membrane capable of blocking infrared rays including heat rays.
However, the agricultural sheet disclosed in JP H09-205898 A may have a poor see-through property and look dark because the inorganic particles chiefly consisting of tin oxide are contained at least 3 or 10 parts by weight in 100 parts by weight of synthetic resin so that visible light transmittance is poor although infrared rays including heat rays can be blocked. Further, long-term weatherability is hardly obtained because ultraviolet rays cause strength deterioration and fading even when an antifouling layer or antisweat layer is provided on the surface of composite resin containing polyethylene terephthalate-based resin, polyvinyl chloride resin or polyolefin resin as a base resin.
JP 2002-69258 A discloses a heat-ray-blocking fluororesin film containing composite particles having 95% distribution range of particle diameter of 0.1-30 μm dispersed in the fluororesin. The employed composite particles of metal oxide coated with amorphous silica have a large particle diameter (average particle diameter: 0.5-10 μm) and, therefore, the transparency may decrease and exhibit the particle color unnaturally when the particles contain enough to block heat rays.
JP 2004-25586 A discloses a composite sheet provided with a fluororesin layer containing near-infrared-blocking inorganic particles on at least one surface of reinforcing matrix. The employed reinforcing matrix made from a fiber woven fabric such as glass fiber woven fabric, polyamide-based fiber woven fabric and polyester-based fiber woven fabric, may have a poor see-through property with colored patterns of the fiber woven fabric.
JP 2001-171060 A suggests controlling sunlight transmittance by providing a three-layer coextrusion-laminated polyester film with an interlayer of polyester resin containing a near-infrared absorbent such as immonium-based compound, phthalocyanine-based compound, aluminum compound and polymethine compound, with visible light transmittance of 70-90% and haze of 5.0% or less. Even by such a method, transmitted light may be colored with the near-infrared absorbent to decrease visible light transmittance. Even a colored film provided with an interlayer containing a near-infrared absorbent and dye having absorbance in the visible ray region may have a further decreased visible light transmittance. Further, the base resin is polyester and, therefore, long-term weatherability is hardly obtained because of deterioration by ultraviolet rays.
JP 2008-181838 A discloses a transparent conductive laminate having a neutral color without yellowish ITO (indium tin oxide) transmission color made by laminating a specific refractive index material and a transparent conductive membrane made of ITO in this order on a transparent substrate. That method makes the transparent conductive membrane (ITO) as the outermost layer so that exterior long-term weatherability may be poor while bending resistance and abrasion resistance may be poor as an inherent problem of the thin-film laminate. Further, it may cost a lot to form the transparent conductive membrane (ITO) by a method such as vacuum deposition method, sputtering method and CVD method, because the processing is complicated.
Furthermore, there may be a problem that the heat-ray-blocking metal oxide typified by ITO added to fluororesin may have a poorer heat-ray blocking ability relative to the initial blocking over an exterior long-term use. As well, long-term weatherability over 3 years of exterior exposure may be poor.
It could therefore be helpful to provide a transparent heat-ray-blocking fluororesin which is excellent in heat-ray blocking while keeping fluororesin film characteristics such as transparency, mechanical properties and long-term weatherability capable of being used exteriorly in the long term. In the specification, the term “heat-ray blocking” means a functionality to totally (or partially) reflect (or absorb) a ray classified to infrared rays so that the ray is prevented from transmitting completely or partially.